Exhaust emission control device

ABSTRACT

An exhaust emission control device reduces nitrogen oxide included in exhaust air from an internal combustion engine. The device includes an exhaust pipe, a catalyst, a supply device, a tank, and a temperature regulating device. The exhaust pipe defines a passage for exhaust air discharged from the engine. The catalyst is disposed in the exhaust pipe. The catalyst is capable of promoting reduction reaction of the nitrogen oxide in exhaust air. The supply device is for supplying a fluid-state additive agent, which is used for the reduction reaction, to an upstream side of the catalyst in a flow direction of exhaust air. The additive agent is stored in the tank. The temperature regulating device is for regulating temperature of the additive agent, which is supplied by the supply device, to be in a predetermined range.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2007-301761 filed on Nov. 21, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust emission control device for reducing a nitrogen oxide included in exhaust air of an internal combustion engine such as a diesel engine, and the invention is effectively applied to vehicles.

2. Description of Related Art

According to an exhaust emission control device for reducing a nitrogen oxide (NOx) included in exhaust air of an internal combustion engine such as a diesel engine, the nitrogen oxide is purified (reduced) by providing in an exhaust pipe a catalyst that promotes a reduction reaction and by injecting an additive agent such as a urea water solution into exhaust air flowing into the catalyst (see, for example, JP2003-293739A).

More specifically, Urea (CO(NH2)2) injected into exhaust air is hydrolyzed by exhaust heat (CO(NH2)2+H2O→2NH3+CO2) to generate ammonia (NH3), which is a reducing agent. Then, the nitrogen oxide is reduced by reaction between the nitrogen oxide and the ammonia through the catalyst.

According to the exhaust emission control device of JP2003-293739A, an amount of the additive agent supplied is regulated by a flow control valve. When temperature changes, viscosity or density of a fluid-state additive agent, such as a urea water solution, changes in accordance with the temperature change. Accordingly, even though a degree of opening or opened duration of the flow control valve is constant, the amount of the additive agent (amount of substance) actually supplied varies with temperature.

When the supplied amount of the additive agent is smaller than a required amount of the additive agent, the nitrogen oxide cannot fully be reduced, and thereby a purifying rate of exhaust air decreases. On the other hand, when the supplied amount of the additive agent is larger than a required amount of the additive agent, the additive agent is consumed more than needed. Accordingly, operation cost of the exhaust emission control device increases.

Thus, when the temperature of the additive agent changes, a difference between the amount the additive agent supplied, which is set as control target value, and the actual amount of supply becomes large. Accordingly, the exhaust emission control device may not be operated efficiently.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to efficiently operate an exhaust emission control device.

To achieve the objective of the present invention, there is provided an exhaust emission control device for reducing nitrogen oxide included in exhaust air from an internal combustion engine. The device includes an exhaust pipe, a catalyst, a supply means, a tank, and a temperature regulating means The exhaust pipe defines a passage for exhaust air discharged from the engine. The catalyst is disposed in the exhaust pipe. The catalyst is capable of promoting reduction reaction of the nitrogen oxide in exhaust air. The supply means is for supplying a fluid-state additive agent, which is used for the reduction reaction, to an upstream side of the catalyst in a flow direction of exhaust air. The additive agent is stored in the tank. The temperature regulating means is for regulating temperature of the additive agent, which is supplied by the supply means, to be in a predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating an exhaust emission control device according to an embodiment of the invention; and

FIG. 2 is a flowchart illustrating characteristic workings of the exhaust emission control device according to the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment is an application of an exhaust emission control device of the invention to a urea SCR (Selective Catalytic Reduction) system of a diesel engine for vehicles. The embodiment is described below with reference to the drawings.

Configuration of the Exhaust Emission Control Device

As shown in FIG. 1, an exhaust pipe 2 defines a passage for exhaust air discharged from a diesel internal combustion engine 1. An SCR catalyst 3 (hereinafter referred to as catalyst 3), which promotes reduction reaction of nitrogen oxide in exhaust air, and a DPF (Diesel Particulate Filter) 4 for capturing particulate matter such as soot contained in exhaust air are provided in the exhaust pipe 2. The DPF 4 is located on an upstream side (engine side) of the catalyst 3 in an exhaust flow direction.

A supply valve 5 is a supply means for supplying a fluid-state additive agent (urea water solution in the present embodiment) used for the reduction reaction to the exhaust pipe 2 on the upstream side of the catalyst 3 in the exhaust flow direction. An additive-agent tank 6 is a tank means for storing the additive agent supplied to the exhaust pipe 2.

An additive-agent pump 7 is a pump means for pumping the additive agent stored in the additive-agent tank 6 to the supply valve 5. A regulator 7A is a pressure regulating means for returning the additive agent into the additive-agent tank 6 when a pressure of the additive agent discharged from the additive-agent pump 7 is higher than a predetermined pressure.

A filter 8 is a removing means for capturing and removing foreign substances in the additive agent. The filter 8 is provided in a pipe 8A leading the additive agent discharged from the additive-agent pump 7 to the supply valve 5. A first heater 9, which heats the additive agent in the pipe 8A when provided with supply of electric power from an in-vehicle battery (not shown), is disposed on an upstream side of the filter 8.

A second heater 10, which heats the additive agent in the additive-agent tank 6 using waste heat recovered from the internal combustion engine 1, that is, cooling water of the internal combustion engine 1 as its heat source, is disposed in the additive-agent tank 6. A flow control valve 11, which regulates an amount of the cooling water supplied to the second heater 10, is disposed in a pipe 10A, through which the cooling water is supplied to the second heater 10.

A first temperature sensor 12A is a temperature detecting means for detecting temperature of the additive agent in the additive-agent tank 6. A second temperature sensor 12B is a temperature detecting means for detecting temperature of the additive agent in the pipe 8A. A third temperature sensor 12C is a temperature detecting means for detecting temperature of the additive agent in the supply valve 5. A fourth temperature sensor 12D is a temperature detecting means for detecting temperature of the cooling water.

The second temperature sensor 12B detects the temperature of the additive agent on the pipes 8A on an upstream side of the first heater 9. The third temperature sensor 12C detects the temperature of the additive agent at an end portion of the supply valve 5 near its injection tip (not shown). The fourth temperature sensor 12D detects the temperature of the cooling water on an upstream side of the flow control valve 11.

An electronic control unit (hereinafter referred to as ECU) 13 is a control means for controlling a degree of opening of the supply valve 5, an energizing amount of the first heater 9, and a degree of opening of the flow control valve 11 The ECU 13 is a widely-known microcomputer including a central processing unit (CPU) 13A, a random access memory (RAM) 13B, and a read-only memory (ROM) 13C. A program for controlling the supply valve 5 and the like is stored in the ROM 13C of the ECU 13.

Detection temperatures of the first to fourth temperature sensors 12A-12D are inputted into the ECU 13. Based. on the detection temperatures, the ECU 13 controls the energizing amount of the first heater 9 and the degree of opening of the flow control valve 11, such that the temperature of the additive agent supplied to the supply valve 5 is in a predetermined temperature range (in the present embodiment, a predefined temperature ranging from 60° C. to a boiling point (103° C.) of urea).

An exhaust temperature sensor 14 is a temperature detecting means for detecting the temperature of exhaust air discharged from the internal combustion engine 1. A NOx sensor 15 is a NOx detecting means for detecting the nitrogen oxide included in exhaust air which has passed through the catalyst 3.

Basic Workings of the Exhaust Emission Control Device

The exhaust emission control device hydrolyzes (CO(NH2)2+H2O→2NH3+CO2) urea (CO(NH2)2), which is the additive agent injected into exhaust air, using exhaust heat so as to generate ammonia (NH3), which is a reducing agent. Then, the exhaust emission control device causes reaction between the nitrogen oxide and the ammonia through the catalyst 3 so as to purify (reduce) the nitrogen oxide.

In order to hydrolyze urea, the temperature of exhaust air may be equal to or higher than 175° C. When the temperature of exhaust air is equal to or higher than 175° C., the nitrogen oxide is efficiently purified (reduced).

Characteristic Workings of the Exhaust Emission Control Device

As shown in FIG. 2, the exhaust emission control device (the supply valve 5 and the additive-agent pump 7) is started at the same time as starting of the internal combustion engine 1. The amount of the additive agent supplied is controlled (hereinafter referred to as normal control) normally, based on the temperature of exhaust air discharged from the internal combustion engine 1, the amount of the nitrogen oxide contained in the exhaust air, and the like.

The control (hereinafter referred to as additive agent temperature control) in FIG. 2 is started at the same time as the normal control and is performed independently of the normal control. According to the control shown in FIG. 2, as described above, in brief, the energizing amount of the first heater 9 and the degree of opening of the flow control valve 11 are controlled, such that the temperature of the additive agent supplied to the supply valve 5 is in the predetermined temperature range The processing is explained below in detail with reference to FIG. 2.

When the additive agent temperature control is started, it is determined whether any of additive-agent temperatures which the first temperature sensor 12A to the third temperature sensor 12C have detected is equal to or lower than a first predetermined temperature T1 (60° C. in the present embodiment) (S1). If it is determined that any of the additive-agent temperatures is equal to or lower than the first predetermined temperature T1 (S:YES), the energization of the first heater 9 is started, and the flow control valve 11 is opened and thereby water starts to flow through the second heater 10 (S2).

If it is determined that all of the additive-agent temperatures, which the first temperature sensor 12A to the third temperature sensor 12C have detected, are higher than the first predetermined temperature T1 (S:NO), or when the energization of the first heater 9 is started and the flow control valve 11 is opened (S2), it is determined whether any of the additive-agent temperatures which the first temperature sensor 12A to the third temperature sensor 12C have detected is equal to or higher than a second predetermined temperature T2 (80° C. in the present embodiment) that is higher than the first predetermined temperature T1 (S3).

If it is determined that any of the additive-agent temperatures which the first temperature sensor 12A to the third temperature sensor 12C have detected is equal to or higher than the second predetermined temperature T2 (S3:YES), the energization of the first heater 9 is stopped, and the flow control valve 11 is closed, thereby stopping the flow of water through the second heater 10 (S4).

If it is determined that all of the additive-agent temperatures which the first temperature sensor 12A to the third temperature sensor 12C have detected are lower than the second predetermined temperature T2 (S3:NO), or after the energization of the first heater 9 is stopped and the flow control valve 11 is closed (S4), the processing SI is performed again after a certain time elapses (S5).

Characteristics of the Exhaust Emission Control Device of the Present Embodiment

In the present embodiment, the temperature of the additive agent supplied to the supply valve 5 is regulated to be in a predetermined temperature range. Accordingly, temperature change of the additive agent is made small.

As a result, a difference between the amount of the additive agent supplied set as the control target value and the actual amount of supply is made small. Therefore, the exhaust emission control device is efficiently operated.

When urea is used as the additive agent, as described above, urea is hydrolyzed using exhaust heat to generate ammonia, which is a reducing agent. Accordingly, when the temperature of the additive agent (urea) is low, the temperature of exhaust air is lowered. Thus, the hydrolysis reaction of the additive agent may be retarded.

In the present embodiment, the temperature of the additive agent is regulated to range from 60° C. to the boiling point of urea. Accordingly, the decrease in the exhaust-gas temperature is limited, and the retardation of the hydrolysis reaction of the additive agent is prevented.

Because ambient temperature such as in the nighttime lowers greatly in a cold district , so that the additive agent stored in the additive-agent tank 6 is frozen or turned into a sherbet form, it is highly possible that the additive agent cannot be supplied to the exhaust pipe 2 particularly at the time of cold starting.

In order to address the above problem, a heating means for heating the additive agent stored in the additive-agent tank 6 would resolve the problem. Nevertheless, when another heating means is newly provided in addition to the second heater 10 for keeping the temperature of the additive agent in the predetermined temperature range, increased manufacturing costs of the exhaust emission control device are caused.

In the present embodiment, however, by heating the additive agent stored in the additive-agent tank 6 using the second heater 10, the temperature of the additive agent is regulated to be in the predetermined temperature range, which is a feature of the present embodiment. Accordingly, the second heater 10 also serves as the above heating means.

As a result, in the present embodiment, the increased manufacturing costs of the exhaust emission control device are limited, and the exhaust emission control device is operated efficiently. In the present embodiment, the second heater 10 uses waste heat of the internal combustion engine 1 as its heat source. Accordingly, the heat source for heating does not need to be newly provided. Therefore, the increased manufacturing costs of the exhaust emission control device are limited, and the exhaust emission control device is operated efficiently.

In the present embodiment, the supply valve 5 corresponds to the “supply means”, the additive-agent tank 6 corresponds to the “tank”, and the first heater 9, the second heater 10, the flow control valve 11, and the ECU 13 constitute a “temperature regulating means”.

OTHER EMBODIMENTS

In the above embodiment, the first heater 9 and the second heater 10 serve as the temperature regulating means. However, the invention is not limited to the above. That is, one of the first heater 9 and the second heater 10 may not be used, or another heater may serve as the temperature regulating means.

In the above embodiment, the energization of the first heater 9 and passing water through the second heater 10 are controlled in a binary manner (ON-OFF). However, the invention is not limited to the above. That is, the temperature of the additive agent may be controlled by continuously varying the energizing amount and the passing water amount.

In the above embodiment, urea is used as an additive agent. However, the invention is not limited to the above. That is, a reducing agent other than ammonia, or an additive agent that generates this reducing agent may be used.

Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. 

1. An exhaust emission control device for reducing nitrogen oxide included in exhaust air from an internal combustion engine, the device comprising: an exhaust pipe defining a passage for exhaust air discharged from the engine; a catalyst disposed in the exhaust pipe, the catalyst being capable of promoting reduction reaction of the nitrogen oxide in exhaust air; a supply means for supplying a fluid-state additive agent, which is used for the reduction reaction, to an upstream side of the catalyst in a flow direction of exhaust air; a tank in which the additive agent is stored; and a temperature regulating means for regulating temperature of the additive agent, which is supplied by the supply means, to be in a predetermined range.
 2. The exhaust emission control device according to claim 1, wherein: the additive agent is urea; and the temperature regulating means regulates the temperature of the additive agent to be a temperature ranging from 60° C. to a boiling point of urea.
 3. The exhaust emission control device according to claim 1, wherein the temperature regulating means regulates the temperature of the additive agent to be in the predetermined range by heating the additive agent, which is stored in the tank.
 4. The exhaust emission control device according to claim 1, wherein the temperature regulating means regulates the temperature of the additive agent to be in the predetermined range by heating the additive agent using waste heat recovered from the engine as a heat source. 